Method of controlling the spread of an adhesive on a circuitized organic substrate

ABSTRACT

A method for preparing a circuitized organic substrate for the subsequent deposition of an adhesive thereon is provided. The method comprises exposing the circuitized substrate to a plasma formed from a gas mixture comprising a fluorine-containing entity. Preferably, the gas mixture used to form the plasma also comprises oxygen. It has been determined that treatment of the circuitized substrate with a plasma formed from a gas mixture comprising at least 20% by volume of the fluorine-containing entity and, preferably, up to about 80% by volume of oxygen reduces the spread of an adhesive deposited on the surface of the organic substrate. It has also been determined that such treatment does not adversely affect the subsequent bonding of wires to the wire bond sites that are present on the surface of the substrate.

FIELD OF THE INVENTION

This invention relates generally to a method of controlling the spreadof an adhesive on a circuitized organic substrate. More particularly,this invention relates to a method of treating the organic substrate soas to reduce the spread of an adhesive that is subsequently deposited onthe circuitized surface thereof

BACKGROUND OF THE INVENTION

Typically semiconductor chips are attached to organic circuitizedsubstrates by means of a polymeric, organic, adhesive. The adhesive,which initially is in the form of a viscous liquid, is distributed ontopreselected regions on the circuitized surface. Thereafter, theperimeter of the semiconductor chip is aligned with and placed on theadhesive. The entire structure, including substrate, adhesive and chip,is then baked to cure the adhesive to thereby strengthen the attachmentof the semiconductor chip to the underlying substrate. In some cases,the semiconductor chip is then electrically connected to the electriccircuitry of the carrier by wire bonding terminals on the chip to wirebond sites that form part of the circuitry on the surface of thesubstrate.

The adhesives that are currently used to attach semiconductor chips tocircuitized organic substrates include for example, epoxy-basedadhesives, acrylic-based adhesives, silicones, and polyimides. Althoughsuch adhesives are fairly viscous, they still have a tendency to spreadover the surface of the underlying substrate, which is formed from anorganic resinous material. Such adhesives also have a tendency to spreadover any portions of a soldermask which lie in the vicinity of thedeposited adhesive. The expanding adhesive may also come into contactwith and cover portions of the wire bond sites that are on the surface,thereby interfering with attachment of the bonding wires to the wirebond sites. The problem of adhesive spread is even more pronounced whenthe circuitized substrate is treated with a plasma containing oxygen,argon or mixtures thereof prior to application of the adhesive to thecircuitized substrate. Such plasma treatment is often used to clean thewire bond sites and to roughen the substrate surface.

Accordingly, a method of controlling the spread of an adhesive over thesurface of a circuitized organic substrate would be highly desirable. Amethod which reduces the spread of the adhesive on the surface of theorganic substrate and which does not interfere with the bonding of thebonding wires to the wire bond sites is especially desirable.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for preparing acircuitized organic substrate for the subsequent deposition of anadhesive thereon is provided. This method comprises exposing thecircuitized substrate to a plasma formed from a gas mixture comprising afluorine-containing entity. Preferably, the gas mixture used to form theplasma also comprises oxygen.

In accordance with the present invention, it has been determined thattreatment of the circuitized substrate with a plasma formed from a gasmixture comprising at least 20% by volume of the fluorine-containingentity and, preferably, up to about 80% by volume of oxygen reduces thespread of an adhesive deposited on the surface of the organic substrate.Such treatment also reduces the spread of the adhesive on the metalliccomponents present on the surface of the substrate. It has also beendetermined that such treatment does not adversely affect the subsequentbonding of wires to the wire bond sites that are on the surface of thesubstrate.

DETAILED DESCRIPTION

In accordance with the present invention, an organic substrate havingwire bond sites plated on a circuitized surface thereof is treated witha fluorine-containing plasma to reduce the spread of an adhesive whichis subsequently deposited onto preselected sites of the substratecircuitized surface. Such treatment prevents the adhesive from spreadingover the substrate surface and from contacting the wire bond sites whichare present on the surface of the substrate. In the preferredembodiment, the substrate is a semiconductor chip carrier.

Any organic resinous material which typically is used to form theunderlying substrate of a semiconductor chip carrier can be subjected tothe plasma treatment of the present invention. Examples of resinousmaterials which are used to form such underlying substrates include forexample epoxy-based resins, epoxy resins reinforced with wovenfiberglass, polyimides and benzotriazol resins.

Any organic material which is used to form a soldermask can also besubjected to such plasma treatment. Typically such soldermasks are madefrom epoxy-based resins, acrylate-based resins, and combinations ofepoxy-based and acrylate-based resins. One example of the material whichis frequently used to form a solder mask is the epoxy/acrylate-basedmaterial Vacrel available from E.I. duPont de Nemours Corporation,Wilmington, Del.

Furthermore, the inorganic materials that are used to form the circuitlines, the ground pads, and the wire bond sites which are on the surfaceof the circuitized substrate can also be subjected to the plasmatreatment of the present invention. Such inorganic materials include,for example, copper from which the circuitry is formed and gold, whichalong with nickel is plated over the copper circuitry to provide a wirebondable-surface and to provide protection against corrosion.

In accordance with the present invention, the circuitized substrate isplaced in a plasma chamber and thereafter a gas comprising afluorine-containing entity is introduced into the chamber. Preferably,oxygen is also introduced into the plasma chamber. The resulting gasmixture, comprising the fluorine-containing entity and, preferably,oxygen is then electrically activated to form a plasma. As used herein,a plasma is intended to encompass all gas mixtures in an excited state.Suitable fluorine-containing entities that are present in the gasmixture include, for example, CF₄, C₂ F₆, and SF₆. Preferably, thefluorine-containing entity comprises at least 20% by volume of the finalgas mixture that is introduced into the plasma chamber. More preferably,the fluorine entity comprises at least 50% by volume of the gas mixture,most preferably from about 65% to about 85% by volume of the gasmixture. Preferably, substantially all of the balance of the gas mixtureis oxygen. Although not necessary, the gas mixture may also contain aninert gas such as, for example, argon or helium and minor components ofnitrogen and water vapor. Preferably, the gas mixture comprises lessthan 40% of the inert gas and less than 5% of the minor components.

The plasma is activated by an electrical field using frequencies of upto 2.5 GHz, wherein the preferred frequency range is from about 30 KHzto about 14 MHz. Preferably, the gas pressure of the plasma is fromabout 70 millitorr to about 400 millitorr, more preferably from about100 to about 180 millitorr. Preferably, the substrate is positioned inthe chamber such that the circuitized surface faces the poweredelectrode of the plasma chamber. The process time for treatment with theplasma is preferably from about two minutes to about ten minutes. Theother operating conditions which are suitable for minimizing the spreadof adhesives on the substrate, such as for example gas flow rate, arevariable and dependent on the apparatus used to perform the treatment.

Following treatment with the fluorine-containing plasma and otherprocessing steps, an adhesive is applied to preselected sites on thesubstrate surface. In those instances where the circuitized organicsubstrate is being used as a semiconductor chip carrier, a semiconductorchip is then bonded to the surface via the adhesive.

In an alternative embodiment, the circuitized substrate is pre-treatedwith a plasma containing oxygen at 90 to 400 millitorr pressure for 1 to10 minutes to remove any organic contaminants which may have accumulatedon the surfaces of the gold-plated circuitry. Thereafter, thefluorine-containing gas mixture is introduced into the plasma chamber.

It has been found that treatment of a circuitized organic substrate inaccordance with the present invention makes the surface of the organicsubstrate less wettable to both water and to the materials contained ina typical uncured adhesive material, such as, for example, a liquidepoxy. It has also been found that treatment of the circuitizedsubstrate in accordance with the present invention results in theformation of a thin film of fluorocarbon on the metallic components ofthe circuitized substrate. As a result, the metallic components whichare on the surface of the circuitized substrate are also less wettable.

It has also been found that treatment of a circuitized substrate with afluorine-containing plasma in accordance with the present invention doesnot diminish the strength of the bonds formed between the wire bondsites on the surface of the substrate and the wires which aresubsequently used to connect the wire bond sites to the semiconductorchip nor diminishes the integrity of this electrical contact.

It has also been found that treatment of a circuitized substrate havinggold-plated areas in accordance with the present invention causes, insome instances, a discoloration of the gold. Such discoloration tends tooccur at somewhat greater frequency at high loads and may be associatedwith high etch rate zones in the reactor chamber. Thus, if necessary,the circuitized substrate is further exposed to a plasma comprising aninert gas, preferably argon, after the treatment with thefluorine-containing plasma to remove the discoloration. Preferably, thegas pressure of the plasma comprising the inert gas is from about100-200 millitorr and the process time is from about two to about fourminutes. Then the circuitized substrate is again treated with afluorine-containing plasma to restore the non-wettability of thecomponents on the circuitized surface. Although the conditions employedduring this second treatment with a fluorine-containing plasma may bethe same as described above, it is preferred that the substrate beplaced in the plasma chamber such that the circuitized surface faces theground electrode rather than the powered electrode.

The following non-limiting examples are presented to further illustratethe present invention.

EXAMPLES Example 1

Test panels comprising a fiberglass-filled epoxy-based substrate havingan epoxy/acrylate-based soldermask covering portions thereof andgold-plated circuitry in the form of ground pads and wire bond sitesplated on the surface thereof were loaded into a plasma reactor obtainedfrom Advanced Plasma Systems, Saint Petersburg, Fla. and sold as Model2400. The APS2400 is a 35 kHz system and is configured to hold sixpanels. The organic substrate was an epoxy sold under the brand nameDriclad by IBM Corporation. (Driclad is a trademark of IBM Corporation.)The soldermask was Vacrel 8130 available from DuPont. All panels werepre-treated for 16 minutes at 140 millitorr with a plasma containingargon and oxygen at a ratio of 3 to 2 remove organic contaminants fromthe gold-plated circuitry.

Following exposure to the pre-treatment plasma, the panels were removedand one test panel was loaded into the plasma chamber such that thecircuitized surface faced the powered electrode, while the opposingsurface faced the ground electrode. Control panels comprising anepoxy-based organic substrate but lacking the gold-plated circuitry wereloaded into the remaining 5 cells. A gas mixture of 75% CF₄ and 25%oxygen was introduced into the chamber to a pressure of 140 millitorr. Aplasma was set up by applying an electric power of 3 KW for fourminutes.

Following treatment with the fluorine-containing plasma, a drop of theliquid epoxy ERL 4299 from Union Carbide was deposited on to twentyeight different spaced-apart sites across the circuitized surface of thetest panel and the static contact angle of the epoxy liquid dropletmeasured using a goniometer. The contact angles ranged from about 62° to90°, with contact angles above 40° indicating a significant improvementin the non-wettability of the organic substrate with respect to theepoxy liquid.

The contact angles of the epoxy liquid ERL 2499 on different sites ofthe gold-plated ground pads were also measured. These measurementsshowed that treatment with the fluorine-containing plasma under thepresent conditions increased the contact angle of the epoxy liquid onthe gold-plated components of the circuitized substrate from a range ofabout 25° to 30° to a range of about 45° to 60°.

A water droplet was placed on the surface of the organic substrate andon the surface of the soldermask and the contact angles between thewater droplet on these respective surfaces visually estimated. Thisstudy indicated that the water contact angle on the substrate surfaceincreased from less than 20 degrees prior to treatment with thefluorine-containing plasma to greater than 100 degrees followingtreatment with the fluorine-containing plasma. Treatment with thefluorine-containing plasma increased the water contact angle on thesurface of the soldermask from about 20 to 35 degrees to about 45 to 60degrees. Thus, the present treatment also reduced the wettability of thesoldermask but to a lesser extent.

The ability of an adhesive to adhere to the surface of the treated panelwas assessed using a fracture toughness test. In brief, a cantileverbeam of 8212 laminate was bonded to the circuitized surface of thesubstrate with Kapton tape to create a crack front. The bonded laminatewas then pulled apart at a constant rate until the bonded area failed.The strain energy release rate was calculated and the failure modedetermined. Cohesive failure occurs when the test beam tears, whileadhesive failure occurs when the bonded laminate comes apart at or nearthe interface between the beam and substrate surface. This study showedthat the present treatment did not significantly alter the strain energyrelease rate. In addition, the failure mode of the treated panel andlaminate remained predominantly cohesive.

ESCA analysis on the gold plated components of the panels indicated thata thin film of fluorocarbon had been deposited on the metallizedcomponents of the panel during treatment with the CF₄ /O₂ plasma. Augerspectroscopy indicated that the thickness of the deposit was in therange of 50 angstroms or less. The effect of this deposit on the bondingof wires to the gold-plated wire bond sites was determined by the wirebond pull test, which indicated that the presence of the deposit did notinterfere with the bonding of wires to the gold-plated wire bond sites.

Example 2

Test panels comprising the same components as described in example 1 andpre-treated as described in example 1 were subjected to plasma treatmentwith a gas mixture comprising 75% CF₄ and 25% O₂ as described in Example1 except that the gas pressure was reduced to 100 millitorr.

The contact angles between a drop of the epoxy liquid ERL 4299 and theplasma-treated organic substrate and gold-plated components weredetermined as described above in example 1. The contact angles of theepoxy liquid with the organic substrate ranged from 34° to about 84°.The effect of treatment under these conditions on the adhesion of thepanel to an 8212 laminate was determined as described above in example 1with substantially similar results. The plasma treatment conditions andthe approximated contact angle of a water droplet with theplasma-treated organic substrate are shown in Table 1.

Example 3

A panel comprising the same components as described in example 1 andpre-treated as described in example 1 was then subjected to a plasmatreatment with a gas mixture of 60% CF₄ and 40% oxygen. The plasmatreatment conditions and the approximated contact angle of a waterdroplet with the plasma-treated organic substrate are shown in Table 1.

Example 4

A panel comprising the same components as described in example 1 andpre-treated as described in example 1 was then subjected to a plasmatreatment with a gas mixture of 50% CF₄ and 50% oxygen. The plasmatreatment conditions and the approximated contact angle of a waterdroplet with the plasma-treated organic substrate are shown in Table 1.

Example 5

A panel comprising the same components as described in example 1 andpre-treated as described in example 1 was then subjected to plasmatreatment with a gas mixture of 40% CF₄ and 60% oxygen. The plasmatreatment conditions and the approximated contact angle of a waterdroplet with the plasma-treated organic substrate are shown in Table 1.

                  TABLE 1    ______________________________________    % (vol)    CF.sub.4     Gas               Test  Total Approx.    in Feed          Time   Pressure                         Flow      Load  Load  Contact    Gas   (min)  (MT)    l/min                              KW   (sq.in.)                                         (sq.in.)                                               Angle (°)    ______________________________________    75    4      140     1.4  3    930   5800  >100    75    4      107     1.8  3    930   5800  >100    60    4      140     1.5  3    930   1850  >100    50    4      132     2.4  3    230   2100  >100    40    4.5    132     2.4  3    230   2100  90    ______________________________________     Test Load = Total surface area of test panels.     Total Load = Total surface area of test panels and control panels.

As shown in Table 1, treatment of circuitized epoxy-based substrateswith a plasma comprising from about 40% to about 75% CF₄ and from about60% to about 25% oxygen significantly decreased the wettability of thesubstrate surface that was oriented to face the powered electrode.

Example 6

Panels comprising the same components as described in example 1 andpre-treated as described in example 1 were subjected to a two stageplasma treatment in which the circuitized test panel was first exposedto an oxygen plasma and then to a plasma formed from a gas mixture of65% CF₄ and 35% oxygen. The plasma treatment conditions and theapproximated contact angle of a water droplet with the plasma-treatedorganic substrate are shown in Table 2.

                  TABLE 2    ______________________________________                                                   Approx.                       Gas             Test  Total Contact         Feed   Time   Pressure                             Flow      Load  Load  Angle    Stage         Gas    (min)  (MT)  l/min                                  KW   (sq.in.)                                             (sq.in.)                                                   (°)    ______________________________________    1    O.sub.2                3      123   2.5  3    930   5800  --    2    65%    3.1    140   1.5  3    930   5800  >100         CF.sub.4    1    O.sub.2                2      127   2.4  3    930   1850  --    2    65%    4      140   2.4  3    930   1850  >100         CF.sub.4    ______________________________________     Test Load = Total surface area of test panels.     Total Load = Total surface area of test panels and control panels.

As shown in Table 2, treatment of epoxy-based substrates first with aplasma formed from oxygen and then with a plasma comprising 65% CF₄ and35% oxygen also decreased the wettability of the substrate surface.

Example 7

A panel comprising a glass-reinforced polyimide substrate havinggold-plated circuitry in the form of ground pads and wire bond sites onone surface thereof was loaded into one of the cells of the plasmareactor model 2400 obtained from Advanced Plasma Systems. The oppositeside of the test panel was covered with Vacrel 8130. The panel waspre-treated for 12 minutes at 110 millitorr with a plasma containingargon and oxygen at a ratio of 3 to 2 to remove organic contaminantsfrom the gold-plated circuitry.

Following evacuation of the pre-treatment plasma, a gas mixture of 65%CF₄ and 35% oxygen was introduced into the chamber at a pressure of 140millitorr. A plasma was set up by applying an electric power of 3 KW forfour minutes.

Following this treatment and removal of the panel from the plasmareactor, a water droplet was placed on the surface of the circuitizedorganic substrate and the contact angles between the water droplet andthe substrate surface visually estimated. This study indicated that thewater contact angle on the substrate surface increased from less than 20degrees prior to treatment with the fluorine-containing plasma togreater than 100 degrees following treatment with thefluorine-containing plasma.

Example 8

Test panels comprising the same components as described in example 7 andpre-treated as described in example 7 were subjected to a plasmatreatment with a gas mixture of 65% CF₄ and 35% oxygen except that thenumber of test panels loaded into the plasma chamber was increased tosix. The water contact angles with the surface of the plasma treatedorganic substrate was visually estimated as described above. The plasmatreatment conditions, load conditions, and the approximated contactangle of a water droplet with the plasma-treated organic substrate areshown in Table 3.

Example 9

Test panels comprising the same components as described in example 7 andpre-treated as described in example 7 were subjected to a plasmatreatment with a gas mixture of 75% CF₄ and 25% oxygen. In addition, thenumber of test panels loaded into the plasma chamber was increased totwelve. In this example, one-half of the panels were oriented such thatthe circuitized surface faced the ground electrode. The water contactangles with the surface of the plasma treated organic substrate wasvisually estimated as described above. The water contact angles on thecircuitized surfaces facing the ground electrode was highly variable andunacceptably low in some regions of the panels. The plasma treatmentconditions, load conditions, and the approximated contact angle of awater droplet with the plasma-treated organic substrate are shown inTable 3.

                  TABLE 3    ______________________________________    % (vol)    CF.sub.4     Gas               Test  Total Approx.    in Feed          Time   Pressure                         Flow      Load  Load  Contact    Gas   (min)  (MT)    l/min                              KW   (sq.in.)                                         (sq.in.)                                               Angle (°)    ______________________________________    65    4      140     2.2  3    960   960   ˜100°    65    4      140     2.2  3    960   5800  90°-120°    75    4.5    145     2.2  3    11,600                                         11,600                                               30°-120°    ______________________________________     Test Load = Total surface area of test panels.     Total Load = Total surface area of test panels and control panels.

As shown in Table 3, treatment of a circuitized organic substrate with aplasma comprising from about 65% CF₄ and 35% oxygen to about 75% CF₄ and25% oxygen increased the water contact angle on the surface of thesubstrate from less than 20 degrees to approximately 90 degrees orgreater. These results also indicated that increasing the load in theplasma reactor lessened the effect of the plasma treatment in some areasof the substrate surface.

Example 10

A panel comprising a glass-reinforced epoxy-based substrate having asolder mask covering portions of one surface thereof and gold-platedcircuitry in the form of ground pads and wire bond sites on the samesurface carrying the solder mask was loaded into one of the cells of theAPS2400 plasma reactor. The organic substrate was comprised of the epoxyresin Driclad. Five control panels covered with Vacrel 8130 but lackingthe gold-plated circuitry were loaded into the remaining five cells. Allpanels were pre-treated for 3 minutes at 130 millitorr with a plasmacontaining argon and oxygen at a ratio of 3 to 2 to remove organiccontaminants from the gold-plated circuitry.

Following evacuation of the pre-treatment plasma, a gas mixture of 50%SF₆ and 50% oxygen was introduced into the chamber at a pressure of 120millitorr. A plasma was set up by applying an electric power of 3 KW forsix minutes.

The effect of the plasma treatment on the wettability of the surfacesubstrate was determined by placing a water droplet on the substratesurface and visually estimating the water contact angle. This studyindicated that treatment of the epoxy-based surface under theseconditions increased the water contact angle from about 30 degreesgreater than 100 degrees.

Example 12

Panels comprising circuitized epoxy-based substrates and exhibitingdiscoloration of the gold-plated ground pads following treatment with aplasma comprising approximately 75% by volume CF₄ and approximately 25%by volume oxygen at a gas pressure of 140 millitorr were subsequentlytreated with an argon plasma at 130 millitorr for 2.5 minutes to removethe discoloration. The panels were then re-oriented in the APS2400plasma reactor such that the surface carrying the gold-plated circuitryfaced the ground electrode. A gas mixture comprising 60% CF₄ and 40%oxygen was introduced into the chamber to a pressure of 140 millitorr. Aplasma was set up by applying an electric power of 3 KW for about 3 toabout 4 minutes.

The water contact angles on the panels subjected to this additional twostage process were estimated as described above in example 1 withsubstantially similar results. While the invention has been described tosome degree of particularity, various adaptations and modifications canbe made without departing from the scope of the invention as defined inthe appended claims.

What is claimed is:
 1. A method of controlling the spread of an adhesive on an organic substrate having a circuitized surface, said method comprising:exposing the organic substrate to a plasma comprising a fluorine containing entity for a time sufficient to decrease the wettability of said circuitized surface to the adhesive.
 2. The method of claim 1 wherein the amount of said fluorine containing entity is at least 20% by volume of said plasma.
 3. The method of claim 1 wherein the amount of said fluorine containing entity is at least 40% by volume of the plasma.
 4. The method of claim 3 wherein the plasma further comprises up to about 60% by volume of oxygen.
 5. The method of claim 1 wherein the amount of said fluorine containing entity is from about 65% to about 85% by volume of the plasma.
 6. The method of claim 5 wherein the plasma further comprises from about 15% to about 35% by volume of oxygen.
 7. The method of claim 1 wherein the fluorine-containing entity is selected from the group consisting of CF₄, SF₆, and C₂ F₆.
 8. The method of claim 1 wherein the organic substrate is a semiconductor chip and wherein the amount of the fluorine containing entity is at least 40% by volume of the plasma.
 9. The method of claim 1 wherein the plasma gas mixture comprises up to about 60% by volume of oxygen.
 10. The method of claim 1 further comprising the following steps:(a) exposing the fluorine plasma-treated substrate to a plasma comprising argon; and (b) then exposing the argon plasma-treated substrate to a plasma comprising a fluorine-containing entity for a time sufficient to decrease the wettability of said circuitized surface.
 11. The method of claim 1 wherein the organic substrate comprises an epoxy-based polymer.
 12. The method of claim 1 wherein the organic substrate comprises a polyimide.
 13. The method of claim 1 wherein the organic substrate is exposed to a plasma comprising a fluorine containing entity for a time sufficient to deposit a fluorocarbon film on the circuitry disposed on the surface of the substrate and to decrease the wettability of said circuitry.
 14. A method of reducing the spread of an adhesive deposited on a surface of a circuitized organic substrate, said method comprising the following steps of:(a) providing an organic substrate comprising circuitry on a surface of said substrate; (b) exposing said substrate to a first plasma comprising a gas selected from the group consisting of oxygen, argon, or mixtures thereof, wherein said substrate is exposed to said first plasma for a time sufficient to remove organic contaminants from said circuitry; (c) then exposing said substrate to a second plasma comprising fluorine and oxygen for a time sufficient to decrease the wettability of the surface of the substrate; wherein said exposure to said second plasma reduces the spread of an adhesive that is subsequently deposited onto the surface of the organic substrate.
 15. The method of claim 14 wherein the substrate is exposed to said second plasma for about two minutes to about ten minutes at a gas pressure of from about 70 millitorr to about 400 millitorr.
 16. The method of claim 15 wherein the amount of said fluorine containing entity is at least 40% by volume of the plasma of step (c).
 17. The method of claim 16 wherein the plasma of step (c) further comprises up to about 60% by volume of oxygen.
 18. The method of claim 15 wherein the amount of said fluorine containing entity is from about 65% to about 85% by volume of the plasma of step (c).
 19. The method of claim 18 wherein the plasma of step (c) further comprises from about up from about 15% to about 35% by volume of oxygen. 